The field of this invention is the de-vulcanization of rubber and more particularly a mechanical and chemical process and composition for such de-vulcanization.
The need to de-vulcanize used rubber stems from at least two considerations. First, waste rubber in the form of used tires is an international environmental and ecological problem and a commercial problem. In particular, every year at least 20 million used automobile tires worldwide become unusable. No more than 30 to 40% of these 20 million tires are subjected to treatment for secondary usage. The remainder, at least 12 to 14 million automobile tires, remains in dumps or is burnt in incinerators and actively pollutes the environment. The hazards of rubber decomposition from used tires in tire dumps are well knownxe2x80x94they include the contamination of the water and soil and the breeding of mosquitoes.
Second, it is necessary to de-vulcanize waste rubber before using it as a starting material for products containing rubber. This is because rubber in its vulcanized state cannot bond effectively with ingredients such as tar, mineral oil, synthetic rubber, sulfur, etc. that are used to form rubber products and because rubber in its vulcanized state cannot be molded effectively into the form needed for the final rubber product. It is necessary to use virgin rubber (rubber that has never been vulcanized) or de-vulcanized rubber as the primary raw material for rubber products. Notwithstanding the widely recognized need to conduct the process of de-vulcanization, and to have a cost effective raw material for use in manufacturing of rubber products, the de-vulcanization processes currently used suffer from many drawbacks and the common use of virgin rubber as the starting material for rubber manufacturing is unnecessarily expensive.
Various known methods of de-vulcanization include thermal, ultrasonic, mechanical, chemical/thermal, biotechnological and microwave methods. See, e.g. U.S. Pat. Nos. 6,129,877, 6,095,440, 5,955,035, 5,891,926, 5,799,880, 5,798,394, 5,731,358, 5,683,498, 5,602,186, 5,284,625, 5,275,948, 5,258,413, 4,506,034, 4,459,450, 4,161,464, 4,129,768 and 4,104,205. All of these known methods either involve large energy consumption, expensive raw materials, complicated equipment, toxic reagants and/or toxic byproducts, have de-vulcanization properties that are not industrially acceptable or in one case is usable only for special types of rubber not including tires. As a result, the cost of known methods of de-vulcanization is approximately $500 to $600 per metric ton of end product, a cost that can be substantially reduced if better methods were known.
The cost per metric ton of producing devulcanized waste ruber for secondary usage varies with the type of product that the waste rubber was originally used as and how it was treated during de-vulcanization. The cheapest de-vulcanized waste rubber is rough rubber crumb and small rubber finders up to 50 millimeters. The most expensive is rubber regenerate from which is produced the most technologically interesting and highest quality rubber products. Regenerate rubber cost up to $600 to $650 per metric ton to produce, according to known methods.
Another disadvantage is that the end product of known de-vulcanization methods is not high quality rubber usable as the starting raw material in the manufacture of rubber products and is used as a filler instead. The starting material for rubber products is raw rubber that was never vulcanized, sometimes called virgin rubber. Virgin rubber, however, requires the addition of many ingredients, e.g., sulfur, tar, carbon black, mineral oil to make usable rubber products. This occurs under conditions of heat during vulcanization. If de-vulcanized rubber could be made more cost effectively and of sufficient high quality then de-vulcanized rubber could be used as the starting process of rubber products in place of virgin rubber without having to add tar, carbon black, sulfur, oil or other ingredients.
Another disadvantage of some methods of de-vulcanization is that it is not long term in the sense that the sulfur remains active in the presence of moderate heat. Consequently, using these methods would cause problems during shipping and storage of the de-vulcanized rubber for later use if exposed to heat from the sun or other sources.
What is needed is a process of de-vulcanizing waste rubber, e.g. used tires, which costs substantially less per metric ton than the known methods of de-vulcanization, produces an end product that is of sufficiently high quality rubber that it can be used as a substitute raw material for virgin rubber in the manufacture of all kinds of rubber products, which uses no toxic reactants and products no toxic byproducts rendering it environmentally friendly and which produces stable enough end product that it does not re-vulcanize-prematurely. This would both revolutionize the cost of making rubber products and greatly enhance the quality of the environment, The method and composition of the present invention achieves these objectives as well as others.
The method of the present invention is a mechanical/chemical method and composition for de-vulcanization of rubber that maintains the macromolecules of waste rubber, renders the sulfur passive for later revulcanization, is cost effective, environmentally friendly and produces high quality de-vulcanized rubber to replace virgin rubber. Waste rubber is shredded, crushed and any metal is removed. Modifying composition is added as the particles of shredded waste rubber are poured between two roll mills that further crush the particles. The modifying composition is a mixture of (1) a proton donor that selectively breaks the sulfur bonds and renders the sulfur passive, (2) a metal oxide and (3) an organic acid that activates new bonds between macromolecules for later revulcanization, (4) an inhibitor that prevents reattachment of sulfur radicals with each other before the proton donor attaches itself to the sulfur and (5) a friction agent that prevents sliding of the waste rubber between rollers of the roll mills. The particles are subjected to at least ten pairs of rollers in at least ten roll mills, examined for consistency and subjected to further sets of roll mills if necessary. It should be understood that a single multi roll mill shall be understood as comprising separate roll mills for each pair of rollers in the multi roll mill. The composition selectively breaks the sulfur to sulfur bonds while allowing the carbon to carbon bonds to remain intact thereby maintaining the integrity of the rubber macromolecules.
The following important objects and advantages of the present invention are:
(1) to provide a method of de-vulcanization of waste rubber such as used tires that is environmentally friendly in that it recycles used tires into raw rubber without using and without emitting any toxic wastes;
(2) to provide a method of de-vulcanization of waste rubber wherein fine grinding of waste rubber is unnecessary;
(3) to provide a de-vulcanization method that creates a stable end-product that does not re-vulcanize prematurely when exposed to moderate heat such as 50 to 70 degrees centigrade;
(4) to provide a de-vulcanization method that is substantially cheaper to produce each metric ton of end product than existing methods;
(5) to provide a de-vulcanization method that provides high quality rubber end product;
(6) to provide-a de-vulcanization method and composition wherein the composition is made of ingredients having a weak smell and that do not have an unpleasant smell;
(7) to provide a de-vulcanization composition and method wherein the composition is made of ingredients that are almost water insoluble;
(8) to provide a de-vulcanization method and composition wherein the composition is made of ingredients that are not inflammable;
(9) to provide a de-vulcanization method and composition wherein the composition is made of ingredients that are cheap;
(10) to provide a de-vulcanization method and composition wherein the composition is sufficiently stable that it remains in usable form for at least half a year,
(11) to provide a composition for de-vulcanization wherein the components are environmentally safe for humans and the environment;
(12) to provide a composition for de-vulcanization of rubber which is added to the crushed waste rubber particles in a ratio that is approximately between 3 parts by weight of composition to 100 parts by weight of waste rubber up to approximately 6 parts composition to 100 parts waste rubber and that is ideally 5 parts by weight of composition to 100 parts by weight of waste rubber;
(13) to provide a rubber de-vulcanization method wherein the cross-linkages from the sulfur bonds are broken but the sulfur remains passively in the rubber and can be reactivated upon later exposure to heat during re-vulcanization thereby eliminating or greatly reducing the amount of sulfur that has to added during re-vulcanization;
(14) to provide a method of de-vulcanization that employs chemical and mechanical means in combination;
(15) to provide a de-vulcanization method wherein the chemical ingredients used react instantly when added to the waste rubber and are not detectable thereafter;
(16) to provide a de-vulcanization method that results in an end product that can be re-vulcanized without the addition of anything;
(17) to provide a de-vulcanization method wherein the rubber end product of the method can be a significant ingredient in the manufacturing of rubber-based products such as asphalt mixtures, bitumen coating, water-resistant paints, rubber glues, anticorrosion mixtures, conveyer belts, tractors and heavy equipment, tires for trucks and tractor trailers, elastic coating, hermetic mixtures, pavements, sports arenas, children""s play areas and car tires;
(18) to provide a de-vulcanization method employing a composition that leaves intact the carbon to carbon bonds and maintains the integrity of the rubber macromolecules;
19) to provide a composition for de-vulcanization that generates an active particle with dimensions not more than 10 angstroms and that is active for 10-100 milliseconds;
(20) to provide a composition for de-vulcanization that conducts a chemical attack on sulfur to sulfur bonds and sulfur to carbon bonds;
(21) to provide a composition for de-vulcanization that generates an active particle such that after the active particle joins the polymer macromolecules of rubber, chemical structures do not appear in place of the sulfur to sulfur bonds that have been broken;
(22) to provide a high quality raw material for rubber vulcanization comprising de-vulcanized waste rubber that contains a composition, in which at least approximately 70% of the sulfur to sulfur bonds that existed prior to de-vulcanization of the waste rubber have been broken, in which less than approximately 10% to 15% of the carbon to carbon bonds that existed prior to de-vulcanization of the waste rubber have been broken, which is stable for at least six months, which may be vulcanized by heating to a temperature of approximately 140 degrees Centigrade without adding sulfur, carbon black, tar, mineral oil and/or virgin rubber.